The present invention relates to a Doppler ultrasound diagnostic apparatus which transmits ultrasonic waves to a an examined part of a subject to be diagnosed, and receives ultrasonic waves from the examined part of the subject, alternately changes over, in a time-sharing manner, the operation for transmitting and receiving ultrasonic waves for obtaining a B-mode tomographic image and the operation for transmitting and receiving ultrasonic waves for obtaining complex Doppler signals in the Doppler mode, in order to measure, for example, the velocity of blood flow by the Doppler method, while observing the B-mode tomographic image of the subject in real time. More specifically, the invention relates to a Doppler ultrasound diagnostic apparatus which is capable of decreasing any deterioration in the quality of the Doppler sound.
Japanese Unexamined Patent Publication No. 5-344971 discloses a method of interpolating Doppler audio signals.
In the method of interpolating Doppler audio signals in the conventional Doppler ultrasound diagnostic apparatus, Doppler audio signals, which are not complex signals, are subjected to interpolation and, hence, the signal waveforms are not often smoothly connected, depending upon the phases of the signals at points where the Doppler signals are folded back, just before and just after the Doppler mode is changed over to the B-mode. This is because, when the data is simply read out in the reverse order, a difference in the amplitude of the signals can be eliminated, but a difference in the phase of the signals can not be eliminated. Therefore, audio signals output in the Doppler mode are distorted, and the quality of Doppler sound is deteriorated. A difference in the phase is frequently generated with an increase in the speed of the change over between the Doppler mode and the B-mode, as in the case where it is intended to improve real time performance of the Doppler ultrasound diagnostic apparatus, and the quality of Doppler sound deteriorates making it difficult to accomplish the diagnosis.